Primary polycythemia is remarkable chiefly for its chronicity and the unusual variations which occur in its course and terminal phases. The disease may be of many years' duration, and the clinical changes may develop so insidiously as to make it impossible to determine the transition from one stage to another; unless this is comprehended, later phases of the condition may appear to be entirely unrelated to the original polycythemic stage.
Any thorough study of the entire course of the disease should include a preliminary consideration of its incipient or developmental phase, which apparently requires several years to reach its peak, although in some instances less time may be required. As no adequate hematologic tests have yet been devised for the diagnosis in this phase, its duration can only be conjectured. And since the blood picture is the diagnostic criterion, it is impossible to determine the presence of the disease until